With some experimenting with a test library, I can provide a little info why you were told not to use it.
It works for MP3’s and OGG files, but doesn’t work for FLAC or M4A files.
FLAC files will use the genre in the embedded tags and apply those to the album, but the Artist will have no genres at all. M4A files will not use the embedded genres for anything, both albums and artist will be genre-less. 
I can foresee the final fix, unfortunately, will be to pick the lowest common denominator, and no files will recognize embedded genre tags for anything, and we’ll be forced to use the genres supplied by AllMusic. It doesn’t look like Plex uses the genres that MusicBrainz has for albums or artists at all.
